TRANSACTIONS OF THE SECTIONS. 85 
its peculiar character. ‘The lower beds, which consist of white grits, having their 
upper parts interstratified with beds of red, blue and black shale, are most abundantly 
developed in the neighbourhood of Langholm, and have a thickness exceeding 300 
yards. Above this lower bed deposits of limestone, ironstone, sandstone, and shales 
of various colours occur, having in some instances a bed of coal about four inches 
intercalated in them. These deposits are best seen at Couthart Burn, near Eccle- 
fechan, where they have been perforated to the depth of about 100 yards in search 
after coal; and in this portion of the representatives of the mountain limestone, 
the lime quarries of the southern part of Dumfries-shire are worked. 
The whole thickness of this second portion of the formation appears to exceed 
150 yards in thickness, and abounds in fossils common to the mountain limestone. 
Above this, a higher series of beds are found, consisting principally of variegated 
grits, having in their lower parts shale beds; these grits are generally of a reddish 
colour, and inthem Producti and Terebratule occur ; and in some places portions of 
coal plants. This higher series of grits is best developed at Brown Muir, Wood- 
cockair, and Repentance Hill, and they seem to have a thickness exceeding 200 yards ; 
the whole representatives of this formation, as it occurs in Dumfries-shire, being more 
than 650 yards in depth. 
These representatives of the mountain limestone appear to occupy a low position 
amongst the deposits which constitute that formation, the whole of the fossils being 
such as to indicate rather that they are equivalent to the great Scar limestone of the 
north of England, than to either the Yoredale rocks or limestone shales ; since, with 
the exception of one or two species of Goniatites which are met with at Kilnhead, we 
find no traces of the fossils which indicate the higher portion of the mountain lime- 
stone formation. 
The district of country occupied by the representatives of the mountain limestone 
is strongly marked, and the contrast between it and the Silurians, which lie to the 
north thereof, is so great, as at once to point out the different formations, the former 
consisting of flat-topped hills of comparatively low elevation covered by coarse herbage, 
while the Silurian hills are round, and have hummocky sides clothed with that fine 
vegetation which so well adapts them for sheep-walks; and to them the south of 
Scotland owes its pastoral beauty. 
On the Erosions of the Earth's Surface, especially by Rivers. By the Rev. 
Epwarp Hircucock, D.D., LL.D., President of Amherst College, Mas- 
sachusetts, and Professor of Geology. 
The evidences of extensive erosions exist in the enormous amount of gravel, sand 
and loam on the surface; in the rounded and striated appearance of mountains in high 
latitudes ; in the marks of erosion in’gorges and valleys, and in the loose materials 
along rivers ; and, above all, in the vast extent of strata that must be supplied to fill 
up deficiencies. 
1. The agents of erosion are—first, atmospheric air; second, carbonic acid ; third, 
water: the latter the grand agent,—1, as vapour; 2, as water ; 3, as ice. 
It (water) operates,—first, chemically as a solvent of great power, at high temperature 
especially ; second, mechanically ;—first, as running water ; second, by its expansive 
power, as ice; third, by the movement of masses of ice, as glaciers or icebergs. The 
results of these agencies are seen—first, in the encroachments of the ocean on the 
land; second, in the erosion of hills and mountains, when the land was beneath the 
ocean, or rising from it. 
2. Long and straight gorges, called durgatories in New England, as in Newport 
R. Island, 7 rods long, 70 feet deep, and 8 feet wide; second, in Sutton, Massa- 
chusetts, half-a-mile long, and 60 to 80 feet deep, and 50 feet wide ; third, in Great 
Barington, Mass., 80 rods long, 60 feet wide, and from 60 to 80 feet deep ; fourth, 
Dixrille Notch, N. Hampshire, 500 feet deep. 
3. Drift Agency, or the joint action of ice and water. 
4, The erosive agency of rivers, which is chiefly considered in this paper. Drift 
agency and sluice action may be distinguished,—first, by the direction of the force; 
second, drift agency has rounded and smoothed the northern sides of hills: it has 
not conformed, like river action, to the sinuosities and anfractuosities of the rocks ; 
